Physicists at Northwestern University in the United States have managed to catch and hold for several months a single particle of antimatter, the positron. This has allowed them to control its state with high precision and has opened the way to an experiment that may explain why our universe is made of matter rather than having vanished at the moment of the Big Bang.
According to current understanding, the birth of the universe should have produced equal amounts of matter and antimatter. But when a particle and its antiparticle meet, they annihilate each other, turning into energy. Had that happened, the universe would simply have disappeared. The existence of our world suggests that matter had some advantage, however tiny. Finding it is one of the central tasks of physics.
Studying antimatter is extremely difficult: it cannot be placed in an ordinary container, because on contact with the walls it is immediately destroyed. A group of scientists led by Professor Gerald Gabrielse solved this problem. They used a positron, the antiparticle of the electron, which was emitted during the radioactive decay of the sodium-22 isotope. With the help of a complex system of electric and magnetic fields, the researchers slowed the fast particle and placed it in a special trap.
Inside the trap, a deep vacuum is maintained and its interior is cooled to 0.3 degrees above absolute zero. Invisible fields hold the positron in place, preventing it from touching the walls. Under these conditions the particle calms down, settles into its lowest-energy state and behaves like an "artificial atom" that can be controlled for months. The scientists call their apparatus the "single-positron quantum cyclotron."
Physicists can now measure the positron's magnetic moment, one of its fundamental characteristics, with enormous precision. In an earlier experiment, a similar parameter for the electron was measured to within one part in ten trillion. The aim of the new study is to compare these two values. The Standard Model, today's main description of the world of elementary particles, predicts that they should be exactly identical.
If the measurements show a discrepancy, this would mean that the laws of physics for matter and antimatter do differ, at least slightly. Such a discovery would be a breakthrough in our understanding of how the universe is built.
Current theories predict that there will be no difference between the two measurements. If they turn out to be unequal, it will be the greatest discovery in physics in the last century.
— Gerald Gabrielse, lead author of the study
The results have been published in the scientific journal Physical Review Letters.
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